An SiC semiconductor is a semiconductor made of SiC (silicon carbide) which is a compound of carbon (C) and silicon (Si). A most distinctive feature of the SiC semiconductor is to have a physical value suitable as a material of semiconductor devices (power devices) used in power electronics. For example, in the case of commercially available single-crystal 4H—SiC, a band gap is 3.3 eV which is three times as wide as the conventional Si semiconductor, dielectric breakdown field strength is 3 MV/cm which is about ten times as high as the conventional Si semiconductor and saturation electron velocity is three times as fast as the conventional Si semiconductor. Further, the SiC semiconductor is better in thermal conductivity, heat resistance and chemical resistance than the conventional Si semiconductor and also has a feature of having a higher radiation resistance than the Si semiconductor. From these features, the SiC semiconductor, particularly MOSFETs (MOS field-effect transistors) of SiC is preferably used for semiconductor devices used in power electronics.
However, it has been conventionally problematic that there are many defects in an interface between a gate insulating film (gate oxide film) and SiC and channel mobility is low in MOSFETs of SiC. Particularly, electron mobility in a bulk crystal is as high as 800 to 1000 cm2/Vs in 4H—SiC, whereas it has been a problem that the channel mobility (Si-face) of MOSFETs of SiC is as low as 10 cm2/Vs.
Further, conventionally, an insulating film has been formed by thermal oxidation of SiC or using a CVD method and an interface between the formed insulating film and SiC has been nitrided such as by NO, NO2 or NH3 gas, thereby reducing defects in the interface to improve the channel mobility. However, the channel mobility (Si-face) of the MOSFET of SiC is as low as 40 to 50 cm2/Vs even if the interface is nitrided and a further improvement in the channel mobility is much-needed.
The channel mobility of the MOSFET of SiC is low because there are many defects in the interface of SiC produced by conventional technologies, i.e. interface state density is high. Due to the low channel mobility of the MOSFET of SiC, an on-resistance value of the MOSFET increases. If the on-resistance value of the transistor increases, power consumption increases.
As described above, despite the fact that the electron mobility in the bulk crystal is originally as high as 800 to 1000 cm2/Vs in 4H—SiC, the channel mobility is reduced due to defects (magnitude of the interface state density) if SiC is incorporated in devices such as a MOSFET. That is, by device integration, the potential of SiC originally having a high electron mobility cannot be utilized at all.
Conventionally, in order to address the above problems, some improvements have been made to the methods of forming a gate insulating film through thermal oxidation, CVD, interface nitridation, and the like to reduce the number of interface defects and thereby improve the channel mobility.
On the other hand, in order to address the above problems, there have also been known techniques focusing on the crystal plane of SiC to improve the channel mobility. Some of the techniques focusing on the crystal plane of SiC will now be introduced.
First, in the fabrication of devices such as DMOSFETs (Double implanted MOSFETs) and UMOSFETs (trenched MOSFETs), generally used is a crystal plane having an off-angle of 4 or 8 degrees in the <11-20> direction with respect to the (0001) Si-plane (Si-face) or the (000-1) C-plane (C-face) in a standard SiC wafer. However, the channel mobility on the (0001) plane or the (000-1) plane is not so high.
A higher channel mobility has been reported not on the (0001) plane or the (000-1) plane but on the {11-20} plane, which is perpendicular to the {0001} plane, but it is known that misalignment of the crystal plane could lead to a reduction in mobility. Using the {11-20} plane thus allows the channel mobility to be improved, although up to about 6 cm2/Vs at the highest.
It is also known that using a crystal plane having an off-angle within the range of 50 to 65 degrees in the <01-10> direction with respect to the {0001} plane allows the number of interface defects to be reduced and thereby the channel mobility to be improved (see Patent Document 1).
It is further known that using the {03-38} plane also allows the channel mobility to be improved (see Patent Document 2). Here, the {03-38} plane is a crystal plane having an off-angle of 54.7 degrees in the <1-100> direction with respect to the {0001} plane.
Using the {03-38} plane thus allows the channel mobility to be improved, although up to about 11 cm2/Vs at the highest.
It is noted that crystal planes and directions, which are crystallographically expressed in numeric characters with a bar overhead, will be expressed in numeric characters with a minus sign (−) placed in front, instead of with a bar overhead, due to limitations of description in the specification, abstract, and claims of the present invention. It is also noted that [ ] will be used to express an individual direction indicating an intracrystalline direction, < > will be used to express a collective direction indicating all equivalent directions, ( ) will be used to express an individual plane indicating a crystal plane, and { } will be used to express a collective plane indicating equivalent symmetric planes. It is further noted that in the accompanying drawings, crystal planes and directions will be in the original crystallographic expression, that is, expressed in numeric characters with a bar overhead.    [Patent Document 1] JPA-2010-040564    [Patent Document 2] JPA-2002-261275